Parasitic protozoa belonging to the genus Eimeria are the causative agents of intestinal coccidiosis, an enteritis which affects birds. This causes significant economic loss, especially to the poultry industry. (For the purposes of the present application, the term "poultry" is taken to mean birds that serve as sources of eggs or meat. It includes, inter alia, chickens, turkeys, ducks, geese, guinea fowl, pheasants, pigeons and pea fowl). Nowadays, coccidiosis is mainly controlled by the use of antibiotic drugs in the feed. The rapid emergence of drug resistant strains (Chapman H. D. Parasitology Today 9, 159-162 (1993)) and the prohibitive costs of development and registration of a novel drug have led to increased interest in the development of an alternative method of control. The development of effective vaccines has therefore been desirable for many years. However only partial success has been obtained.
Currently available vaccination strategies consist of controlled infections with either virulent or live attenuated parasites (Shirley M. W. In: Proceedings of the Vlth. International Coccidiosis Conference (Eds.: J. R. Barta and M. A. Fernando) Moffitt Print Craft Ltd., Guelph. pp. 61-72 (1993)). For reasons of safety and cost, the most desirable method of immunoprophylaxis against coccidiosis appears to be the use of a subunit vaccine. Although many attempts have been made to immunise chickens against coccidiosis with fractions of parasite material (Murray P. K., Bhogal B. S., Crane M. S. J. & MacDonald T. T. In: Research in Avian Coccidiosis. Proceedings of the Georgia Coccidiosis Conference (Eds.: L. R. McDougald, Joyner L. P. and P. L. Long) Athens, University of Georgia. pp. 564-573 (1986), McKenzie M. E. & Long P. L. Poultry Science 65, 892-897 (1986)) or recombinant Eimeria polypeptides (Danforth H. D., Augustine P. C., Ruff M. D., McCandliss R., Strausberg R. L. & Likel M. Poultry Science 68, 1643-1652 (1989), Jenkins M. C., Augustine P. C., Danforth H. D. & Barta J. R. Infection and Immunity 59, 4042-4048 (1991)) only limited protection against challenge infection could be achieved. The parasite stages responsible for the induction of protective immunity are generally thought to be early asexual developmental stages (Jenkins et al. 1991). Initially, selection of candidate antigens was performed using antibodies from immune chickens but, in view of the fundamental role of cell mediated responses in protective immunity (reviewed in Lillehoj H. S. & Trout J. M. Avian Pathology 22, 3-31 (1993), Rose M. E. In: Poultry Immunology (Ed.: T. F. Davison, T. R. Morris and L. N. Payne), Carfax Publishing Company, Oxfordshire, U. K. pp. 265-299 (1996), attention has now focused, next to B-cell inducing antigens, on screening antigens for their ability to stimulate specific T-cell responses (Dunn P. P. J., Billington K., Bumstead J. M. & Tomley F. M. Molecular and Biochemical Parasitology 70, 211-215 (1995)).
It is an objective of the present invention to provide polypeptides that are capable of inducing protection against the pathogenic effects of Eimeria infection in poultry.